Vinyl chloride is prepared by cracking ethylene dichloride vapor at elevated temperature and pressure in a pyrolysis zone. The ethylene dichloride can be prepared by reacting ethylene with chlorine in a low or high temperature reaction such as disclosed in U.S. Pat. No. 4,347,391. The ethylene dichloride can also be prepared by the oxychlorination reaction of ethylene, a source of oxygen and hydrogen chloride. Each reaction does not form pure ethylene dichloride. The reaction products include heavy ends such as 1,1,2-trichloroethane, 1,1,1,2- or 1,1,2,2 tetrachloroethane, pentachloroethane and light ends such as air, hydrogen chloride, hydrogen, chlorine, ethylene, low boiling chlorinated hydrocarbons and the like. The ethylene dichloride is purified to at least about 98% purity by distillation. Various distillation procedures are available and are well known to those skilled in the art.
Two general distillation methods are used. The first uses sequential reflux columns to remove first the light ends and non-condensables (light ends column) and second to separate the ethylene dichloride from the heavy ends principally 1,1,2-trichloroethane (heavy ends column). In an alternative method such as shown in U.S. Pat. No. 4,347,391, controlled steam flow in a single column allows the light ends and non-condensables to be removed out of the top of the column, the ethylene dichloride to be concentrated and can be taken off as a liquid in the center of the column or as a vapor and the heavy ends to be removed from the bottom of the column.
The ethylene dichloride vapor is generally taken from the distillation column at a temperature within the range of about 85.degree. C. to 120.degree. C. at a pressure of atmospheric to about 2.0 kilograms per square centimeter gauge. The vapor is condensed, the product cooled to about 40.degree. C., and pumped into a storage tank. The product is pumped from the storage tank to the pyrolysis furnace. The ethylene dichloride is vaporized by applying heat, usually in the form of steam, in a heat transfer means to elevate the temperature of the vapor to about 185.degree. C. and a pressure of about 10.0 kilograms per square centimeter gauge. An optional preheater system has been employed whereby the ethylene dichloride liquid is passed through a coil heated by the exhaust gases of the pyrolysis furnace or by heat interchange with another process fluid. In each instance, the preheating and vaporization of the ethylene dichloride to the proper temperature and pressure for entrance to the pyrolysis zone requires approximately 340 kilocalories per kilogram of vinyl chloride monomer produced.